Computed tomography (CT) systems comprise an x-ray tube that emits radiation that traverses an examination region and an x-ray detector that detects the radiation. The x-ray detector generates a signal indicative of the detected radiation, and the signal is reconstructed to generate volumetric image data indicative of the examination region.
The x-ray tube comprises a cathode and an anode. The cathode has included a filament that provides electrons that are accelerated towards and strike a target region on an anode under relatively high voltage. The interaction of the electrons with the anode material at the target region produces radiation that is emitted from the target region. The target region has been referred to as the focal spot. A collimator has been used to collimate the emitted radiation so that a radiation beam traverses the examination region.
U.S. Pat. No. 8,331,529 B2 discloses a medical imaging system with an x-ray source having a focal spot that emits radiation that traverses an examination region. The position of the focal spot along a longitudinal direction is a function of a temperature of one or more x-ray source components. The system further includes a detector that detects the radiation and a collimator disposed between the x-ray source and the examination region that collimates that radiation along the longitudinal direction. A focal spot position estimator dynamically computes an estimated position of the focal spot along the longitudinal direction based on the temperature of one or more x-ray source components. A collimator positioner positions the collimator along the longitudinal direction based on the estimated focal spot position prior to performing a scan.
It is well known that an x-ray output of the x-ray source or tube will decrease with usage. This is caused by thermal stress on the anode creating a roughened surface of an x-ray emitting area of the anode. The anode roughness in particular results from increased anode self-filtration as well as from excessive scattering of x-rays away from an exit window. However, thermal stress and temperatures are not uniform across the x-ray emitting area.
The decrease of x-ray output of the x-ray source is therefore a function of the history of temperature and time of the focal spot and the focal track as well as the physical mechanical properties of the anode. This is further complicated by the fact that in the x-ray tube there exist multiple focal spot sizes as well as dynamic focal spot switching. So the track wear depends on the separate history of focal spot size usage and the dynamic switching used. The x-ray output will also depend on the x-ray spot size used as well as the dynamic switch mode.